Significance Organisms have evolved in the earth's gravitational field. Changes in this gravitational environment affect many of the body's organ systems. For example, in the microgravity of space muscle and bone tissue are lost while, in contrast, at G-levels higher than that of the earth, production of some of these tissues is enhanced. Changes in G-level also affect circadian rhythms. Misalignment between internal body rhythms has been linked to reduced performance of simple tasks and psychological and sleep disorders. Objectives This project examines the effects of a 2G gravitational environment on muscle tissue, performance, and circadian rhythms of brain temperature, ECG, activity, drinking and performance in male rhesus monkeys. Data from this study will be compared with that from a recent space flight experiment. Results We are training our subjects and setting up the equipment that will be necessary for us to conduct this study. Sixteen male rhesus monkeys have been trained to use the Psychomotor Test System (PTS). PTS is a computer based series of video tasks that is designed to test various performance parameters such as eye-hand coordination and short-term memory. In addition, these rhesus monkeys are being trained to walk on a slow-moving treadmill. We will record the activity from the leg muscles used in walking and simultaneously videotape the animal's stride. We will then be able to correlate the force the animal exerted with its movement. Future Directions We will continue our training and preparations for this experiment. Six rhesus will be exposed to a 2G force environment on a large diameter centrifuge. Six animals will remain at 1G and serve as controls. The effect of the hyperdynamic field on skeletal muscles, circadian rhythms and performance will be determined. KEY WORDS Circadian rhythm FUNDING NASA NAG2-1265